This invention relates to a pressure release valve fitted to a container, and more specifically to a gas pressure release valve which allows automatic release of pressure within the container in the event of said pressure rising above a predetermined level, yet remain closed when the container pressure is below said predetermined level.
Although this invention refers almost exclusively to a pressure valve for a road or rail tanker carrying volatile liquid, and particularly to a road tanker for transporting petrol, it will be appreciated by those skilled in the art that the invention could also include other storage arrangements for the containment of any volatile liquid or hazardous chemical. Indeed it is already been proposed to use valves of the present invention to replace those valves already in existence on vent stacks of static tanks beneath petrol garages.
Pressure valves are commonly employed on the top of containers used for the storage or transportation of volatile liquids, particularly on the top of tankers for the transportation of petrol. Volatile liquids evaporate by definition, and the vibration and movement of a volatile liquid within a tanker is often sufficient to cause such evaporation. Furthermore, slight increases in the ambient temperature can have a substantial effect on the degree to which evaporation of a volatile liquid occurs. In any event, evaporation of a volatile liquid in a closed container results in a build-up of pressure in that container. This is clearly disadvantageous, especially when the volatile liquid is highly inflammable, and in extreme circumstances the increase in pressure could result in an explosion.
Pressure release valves are therefore incorporated in those containers to ensure automatic venting of pressure within the container before said pressure rises to a dangerous level. The valve acts to maintain a tolerable pressure within the container until the pressure reaches a predetermined level at which point the valve automatically opens to release the pressure from within the container. When the pressure within the tanker falls again to a tolerable level, the valve then automatically closes to return the container to a concealed state thereby preventing unnecessary release of hydrocarbons into the environment.
Conventionally, pressure valves on tankers consist of a plunger which rests on a valve seat, the action of gravity on the plunger and the corresponding reaction of the valve seat being generally sufficient to form a seal therebetween. Additional springs may bias the plunger towards and away from the valve seat, and accordingly a desired pressure setting of the valve may be achieved such that venting occurs once the pressure within the container reaches a predetermined level.
In practise however, as the pressure within the container increases above the ambient pressure outside the container and thus exteriorly of the plunger of the valve, the plunger begins to rise and any seal between the valve seat and the plunger is broken. This results in unnecessary release of petrol vapour into the environment. The conventional valve is in fact so unreliable that periodic testing of the valves currently provided on petrol tankers almost invariably shows that the valve is faulty which results in the routine replacement of all such pressure valves on that tanker. This inevitably results in increased expense to the owner of the petrol tanker.
In addition to the unnecessary venting of petrol stored in tankers, the possibility of petrol spillage from the tanker through the pressure valves in the event of an accident is also an important consideration.
Conventional pressure valves on the top of tankers have a roll-over mechanism, which acts to seal off the pressure valve in the event of a tanker rolling over, as is common in an accident. When the tanker is overturned, the pressure release valve is automatically shut off by said roll-over mechanism, and the liquid is therefore prevented from leaking out. However, in the event of an accident, there is a requirement to drain the inflammable fluid from the tanker, and such a feature is not currently provided on existing valves. Henceforth, spillage of the inflammable liquid is almost inevitable as a suitable hose is connected to the pressure valve to drain fluid therethrough. It can be appreciated that such spillage of a flammable liquid is potentially lethal.
There are disclosed in WO9857082, FR2252523, U.S. Pat. No. 4,489,754, DE1229927 and U.S. Pat. No. 4,874,012 a number of valves incorporating magnets as a means of attracting or repelling different portions of the valve. In particular, WO9857082 discloses a magnetic annular valve seat which magnetically retains a spherical ball valve in a closed seal against the seat when the fluid pressure beneath the ball valve is below a predetermined value. FR2252523 describes a magnetic valve having an outer shell from which a cruciform structure depends and having an annular plastic insert therein which provides an annular valve seat against which a ferromagnetic disc held within the cruciform structure contacts to close the valve. The disc is magnetically attracted to a permanent magnet mounted in the middle of the shell between arms which traverse said shell. The remaining documents also describe magnetic valves, but only DE1229927 discloses a valve having one valve component magnetically attracted to the other and which also includes a manual release for displacing the valve component out of a sealing condition to re-open the valve.
It is an object of the invention to provide a pressure valve for a tanker or storage container which allows venting of a contained product only when the pressure within the tanker or container reaches a pre-determined level to prevent the unnecessary release of evaporated product into the environment.
It is a further object of the invention to provide a pressure release valve through which a tanker or storage container can be both vented and drained without resulting in spillage of the liquid contained thereby, notwithstanding the particular orientation of the said container.
According to the invention there is provided a pressure release valve comprising a first movable portion and a fixed portion, said valve allowing fluid to flow between the first movable portion and the fixed portion when the first movable portion adopts a first disposition with respect to the fixed portion, and prevents fluid flow when in a second disposition, said first movable portion capable of moving relative to the fixed portion when a pressure differential across the valve reaches a predetermined value, one or other of the first movable or fixed portion having magnets which magnetically attract the alternate portion with a force which is sufficient to prevent relative displacement of the portions when the pressure differential across the valve is below said predetermined value and characterised in that the first movable portion is provided with manual release means comprising a vacuum seal plate which sealingly contacts the first movable portion and can be manually moved relative thereto to break the seal to allow fluid flow through the valve regardless of the pressure differential thereacross.
Preferably, a rollover mechanism is additionally provided underneath the said valve which acts in the event of a rollover to displace a seal plate underneath the valve into sealing contact with a shoulder underneath said valve, and further characterised in that said manual release means communicates with said seal plate to displace same and break both the seal formed between the seal plate against and the valve shoulder and between the vacuum seal plate and the first movable portion of said valve.
Preferably the magnetic attraction between the portions is adjustable such that relative displacement of the portions may be prevented when the pressure differential across the valve is below a variety of predetermined values.
Preferably the valve comprises a movable plunger and a fixed seat having at least a partial ferromegnetic characteristic, said plunger being provided with magnets therein or being magnetised so as to magnetically attract the seat.
Preferably the seat is annular and either plunger or seat comprises an elastomeric seal compressed between the said seat and said plunger thus sealing the valve and preventing fluid to flow therethrough when the pressure differential there-across is below one of said predetermined values.
Preferably, the valve is provided as a vent for a closed container adapted to carry volatile liquid.
Most preferably, the valve is incorporated in a petrol tanker.
In a most preferred embodiment the magnets are sintered neodymium-iron-boron magnets but it is to be noted that a number of other types of magnets can be used that will give a similar performance.
Ideally, the magnetic attraction between the portions diminishes substantially as the portions are displaced relative to one another. This not only ensures that the valve allows fluid to flow therethrough only when the pressure differential thereacross is at precisely the predetermined value and not at some lower value, but also ensures that once the magnetic attraction has been broken, and the portions moved relatively to one another, the valve allows a greater flow therethrough. Prior art valves rely on gravitational and spring bias effects to close the valve exert a constant and/or linearly increasing restoring force on the respective portions dependent on their relative displacement, whereas in the valve of the present invention, the restoring force between the portions is substantially diminished as they are moved apart, with practically only gravitational effects on the plunger acting to restore the valve to its closed condition. Henceforth, far superior venting can be achieved with the valve of the present invention.
In a preferred arrangement, and in circumstances where the predetermined pressure differential across the valve must be particularly high before venting occurs, the valve seat is magnetised or is provided with magnetic means which attract either or both of the magnets in the plunger and the plunger itself.
Most preferably in the case where the valve is to be used by a petrol tanker, the magnetic attraction is broken and the valve portions separate when the pressure differential across the valve is 70 m/bar.
Preferably, said pressure valve has a roll-over mechanism incorporated therein, as described in our co-pending application of even date.